A turbomachine comprises a compressor, a combustion chamber, and a turbine. The compressor serves to increase the pressure of the air delivered to the combustion chamber. The turbine serves to deliver rotary drive to the compressor by extracting a fraction of the pressure energy from the hot gas that leaves the combustion chamber, and by transforming it into mechanical energy.
The compressor and the turbine are made up of a first set of parts that are stationary, constituting a stator, and a second set of parts that are suitable for being set into rotation relative to the stator, and constituting a rotor.
The rotor of the compressor and the rotor of the turbine form a unit that is securely interconnected by means of a rotary shaft. In the event of the shaft breaking, the mechanical connection between the rotor of the compressor and the rotor of the turbine is lost. Under such circumstances, the rotor of the turbine is suddenly subjected on its own to all of the energy coming from the combustion chamber without being able to transmit any of it to the rotor, and it therefore starts to race. The consequences of such racing or “overspeed” can be dangerous and can lead very quickly to the turbomachine self-destructing.
In order to contain such overspeed, various actions need to be taken urgently: firstly the fuel supply must be switched off to interrupt the supply of energy; and secondly the power to which the turbine rotor is subjected needs to be dissipated so as to reduce it. Dissipation can be achieved by deformation, friction, or destruction of the turbine rotor and/or the turbine stator. Examples of techniques for dissipating turbine rotor energy are described in the following documents: U.S. Pat. Nos. 4,498,291; 4,503,667; and 4,505,104.
In order to take urgent action effectively, it is necessary to detect as quickly as possible that the shaft has broken and to forward this information quickly to those members of the turbomachine that enable the urgent action to be taken, and in particular those that enable the fuel supply to be cut off.
One known device for detecting breakage of the turbomachine shaft consists in using a pair of speed sensors, each sensor being disposed close to a respective upstream or downstream end of the shaft. When the shaft is intact, the speeds measured by the two sensors are practically identical. An excessive difference between the speed of the compressor rotor as measured by the upstream speed sensor and the speed of the turbine rotor as measured by the downstream speed sensor is interpreted by a speed comparator as indicating that the shaft has broken. The comparator can then trigger the urgent actions that are required in order to avoid turbine rotor overspeed. Examples of such a device for detecting shaft breakage are described in documents GB 2 256 486 and U.S. Pat. No. 6,494,046.
Nevertheless, implanting a speed sensor close to the downstream end of the turbomachine rotor shaft is difficult since the turbine environment is severe because it is exposed to very high temperatures, given that the turbine is located immediately downstream from the combustion chamber.
Furthermore, using a plurality of speed sensors leads to a detector device that is relatively complex and increases the overall weight of the turbomachine. It is known that reducing weight is a constant concern in aviation.